1. Field of the Invention
The present invention relates generally to electrical testing circuitry for integrated circuits (ICs) and in particular to apparatus for electrostatic discharge (ESD) sensitivity testing that includes a pulse generator (pulser).
2. Description of the Related Art
Testing of ICs for ESD sensitivity involves applying stress pulses simulating electrostatic discharges to an IC or device under test to determine robustness. The use of this stress testing is expanding to help ensure reliability of products such as computers and cell phones when they are exposed to ESD. Equipment that must operate in harsh and/or high static environments, such as automotive electronics, also requires thorough ESD reliability testing.
All electrical equipment sold in the European Community is required to conform to government safety regulations. Products in compliance with such safety regulations may be signified by the CE Mark. One of the many tests for CE Mark compliance is the ability to withstand electrostatic discharges to the surfaces of the product without damage. The International Electrotechnical Commission test method IEC 61000-4-2: Electromagnetic compatibility (EMC)—Part 4-2: Testing and Measurement Techniques—Electrostatic Discharge Immunity Test Ed. 1.2 b: 2001 is a standard that prescribes the ESD test method. It is a purpose of this invention to provide testing similar to this standard but with advantages not found in prior art methods for ESD testing of ICs.
The International Electrotechnical Commission IEC 61000-4-2 testing method, herein termed the IEC Test, defines a testing environment and a discharge gun pulse generator for ESD testing. This equipment and procedure is commonly used for testing a finished product, which is generally termed equipment under test (EUT). This testing method was designed only for testing complete systems, and is not directly applicable to testing of subassemblies, such as circuit boards, or of individual IC components or devices, often collectively termed a component under test (CUT). IC design and/or manufacturing companies have begun testing their products using variations of the IEC Test. The desire to test CUTs with the IEC Test is motivated by the assumption that when a product is assembled from parts that each individually pass the IEC Test, the completed product or EUT will pass the IEC Test.
Testing a CUT with tools and methods designed for an EUT is difficult. A primary reason for difficulty in applying the IEC Test to circuit boards or IC devices is the inability of the IEC Test specified ESD pulse generator to be connected to the typical IC device. The apparatus for ESD event simulation described in the IEC Test standard is a hand-held discharge gun with a large metallic probe tip. These gun tips are shown in FIG. 1. Such prior art ESD pulse generators using a metallic probe tip are difficult to connect to the small dimensions of today's microelectronic circuitry. Gun testing is especially difficult with high density packaging of modern ICs, which may have pin-to-pin spacing of 0.5 millimeters or less. Improper connection leads to pulse distortions that can adversely affect the testing results.
Connection from the IEC Test discharge gun to a CUT has been attempted in a variety of ways, and often with inconsistent results. The ground connection of IEC Test guns are not made with wide bandwidth cables so the return path may be high impedance at pulse frequencies depending on electrical coupling between CUT and the gun. IEC Test guns are known to generate unwanted electromagnetic (EM) radiation pulses in addition to the desired conducted current pulse. These unwanted EM radiation pulses complicate the testing environment, and EM pulse shielding is sometimes done to help mitigate the effect. EM pulse variations may be exacerbated by the placement of conductive surfaces in the testing environment such as the horizontal, vertical and ground coupling planes described in the IEC Test specification (these surfaces can provide a low impedance radio frequency (RF) grounds and reflections of the gun's EM pulse). Given these factors, a good deal of uncertainty can be introduced into IC test results when using IEC Testing discharge guns.
Prior art efforts to test CUTs to the IEC Test have focused on providing conductive paths from the discharge gun tip to one connection on the CUT and from another CUT connection to an earth connected ground plate so that the current produced by the pulse generator passes through the CUT. One prior art test method uses a test fixture board (TFB) to hold the CUT, either in a socket or by soldering the CUT directly to the TFB printed wiring, to make such connections. Electrical terminals, sometimes termed test points, are provided on a TFB that are compatible connections for the discharge gun tip. Printed wiring on the TFB conducts the discharge current from the gun's test point to the CUT and another electrical path for ground return current. This approach suffers from pulse shape distortion due to different electrical properties of the gun and the TFB. The gun is defined by the IEC Test specification to have 330 ohms of series resistance, as shown in FIG. 2. The wiring of TFBs can have impedances from about 30 to 150 ohms, but cannot be manufactured at 330 ohms due to physical printed wiring size limits. The mismatch of impedances will result in pulse reflections and waveform distortions to the fast current pulses. Such prior art testing also suffers from non-reproducibility due to TFB-to-TFB design variations in the conductive paths to the CUT. TFB wiring can act as an antenna picking up energy from the EM pulse, and any variations in the TFB wiring can change the EM pulse coupling from the gun to the TFB. Maintaining the proper waveform is required to simulate ESD events and meet the IEC Test requirements, and variations produce non-repeatable measurements.
Another prior art technique is to eliminate the intermediate conduction paths and antenna properties of the TFB by touching the gun tip directly to the CUT. With the miniature contacts on small microelectronic components, this can be very difficult. A prior art solution is to employ a robot that can position the discharge gun with accuracy that cannot be obtained from a human operator. However, since a complete electrical conduction path also requires a ground return, two connections must be made to the CUT. Even when more than one robot is used it can be impossible to make contacts to neighboring pins due to the size of the IEC Test gun's metallic tip. ESD testing requires specific characteristics from pulse generators to properly simulate an ESD event. Some pulse generators used for bench testing of electronic systems can generate arbitrary waveforms into 50-ohm coaxial cables. In theory, therefore, such pulse generators could generate the waveform that is needed for the IEC Test, and be an alternative to the hand-held gun. However, the IEC Test requires many amperes of current and hundreds, or even thousands, of volts to reach desired stress testing levels. These requirements for high power levels are far beyond the capabilities of common arbitrary waveform generators. A different type of pulse generator is needed for this application.
Prior art test methods lack the power, reproducibly and consistency in applying stress pulses to ICs or other devices that is needed for repeatability in testing. Therefore, there is a need for new equipment and methodologies to produce a repeatable and reliable test to evaluate the ESD sensitivity of ICs, devices, components and subassemblies with a test method similar to the EUT IEC Test but compatible with testing CUTs.